This has been an expertise for awhile, there are quite a few people on the server who have dealt with it, but it was back when hardly anyone played the race and only one or two people had the expertise so it wasn't that big of an issue to deal with. We now have 5 or so people who have been playing the race on a more regular basis and who purposely try to get on teams together to play in a group with 3 of those people for sure having the expertise.
This ^^^ is the exact opposite of what an expertise should be. Expertises were meant to be small little bonuses that players got for dedicating time to a race they enjoy. And there are definitely a lot of races that have some expertises that don't fit that, but making the argument that the expertise is extremely powerful and talking about how it changes the way the race is played so much you give more reason for something to be done about the expertise.
I completely disagree with this, it's not that it's a unique mechanic to Marine that people just need to learn to deal with. We are all very aware of what the race does, we knew about this before you played on this server and before you played the race, we just never had so many people playing the race and using the expertise together. Cache is a perfect example, we had as many people playing counters to marine as possible, but with 3 marines each with the expertise and all being supported by Holy Paladin and Priest, it's not that easy to shut down that comp. The races that counter respawns have a hard time keeping up with that many respawns, and even a race that would counter marine can only do so much when there 3 of them being focused by support races. The T team had 6 people at one point actively trying to counter that comp but were still having a hard time doing it not because they don't understand the mechanic, but because it's a lot to deal with.
As stated above Expertises were always meant to be small bonuses, not huge buffs or things that change the way the race plays and that is exactly what the Marine expertise does. And there are a lot more races that get attacked for there expertises than you think, whether the expertise is good or not. Some just work really well with the race even if they aren't inherently strong. People complain about NE expertises all the time even though none of it's expertises are near as strong as some of the others on races. Meanwhile I think Naix has one of the top 5 strongest ones because of how well it works for it race and almost no one ever complains about that one. It's not a case of marines expertise being noticeable moreso than other races expertises, it's the strength of the expertise and the way it changes the race PLUS the fact you can have multiple people running around with such a strong expertise. Also don't forget that you get to save minerals/respawns ACROSS MAPS and literally no other race gets to do anything like that.
Yet again to this point I will say that it's not a matter of players being unaware of the mechanic, and most people don't think that Marine is OP in and of itself. Marine has the most drastic change of strength for a race when it gets an expertise and that's what causes the issue here, especially when that strength is multiplied for every other marine you add to a team that has the expertise. You yourself recognize that most other respawn races are restricted to a lower amount than marine is AND they have less respawns than it does because of a single expertise.
The three player limit is 100% not unique to Marine, Star Lord is also a three per team limit and I am pretty sure there might be one more. I also don't think that they intentionally made it have that number of respawns because of Starcraft lore, you're thinking way too much into it.
I get wanting to defend your favorite race and not see it get nerfed or touched, I fight tooth and nail for Night Elf almost every single time someone has asked for a nerf or it has actually been nerfed, but you have to be able to recognize when something needs to be done. Players can't always dedicate themselves to watching your dead bodies in case of a respawn or focusing all of their attention on being ready for it and people don't want to always have to be playing a counter for an expertise rather than the race itself. The fact you think that people's issue is that they just don't understand the mechanic or aren't used to it is way off.